1. Field of the Invention
The present invention relates generally to semiconductor based photon detectors and, more specifically, to photodiode pixel sensors.
2. Description of the Related Art
The following descriptions and examples are given as background information only.
Sensors used for detecting electromagnetic radiation (a.k.a., photons) are generally referred to as pixel sensors and are widely fabricated within semiconductor-based materials for applications including but not limited to optical pointing devices and imaging arrays. Semiconductor based pixel sensors commonly employ photodiodes as sensing elements and, in some cases, include a number of active semiconductor devices, such as transistors, to amplify the charge that is collected in a light sensitive element or region in the photodiode. The inclusion of such active devices, however, may undesirably increase the size of a pixel and reduce the collection efficiency of the photodiode. In addition, conventional pixel sensors may be limited in their collection efficiency and accuracy by their architecture, regardless of whether active devices are included. In particular, some conventional photodiode pixel sensor architectures are susceptible to having photon-generated electrons recombine within the semiconductor substrate before being collected by the photodiode. In addition, some conventional photodiode pixel sensor architectures lack sufficient isolation between adjacent pixels, allowing electrons generated in one pixel to be collected in the photodiode of an adjacent pixel. As a consequence, the pixel in which the electrons are generated may experience image loss.
Moreover, some conventional photodiode pixel sensor architectures are susceptible to dark current, which is leakage current collected by the diode when there is no light falling on the detector. Dark current undesirably promotes obscure images with shadows and/or with a “snowy” appearance such as one may see when viewing a bad television transmission and, therefore, is preferably minimized. Dark current is in part due to surface generation leakage current as well as the generation of carriers in the depletion region of a diode in the absence of light. In some conventional photodiode pixel sensor architectures, the depletion region of the diode may be in close proximity to the surface and, therefore, the generation of carriers at the surface may substantially increase the generation of carriers in the depletion region, resulting in increased dark current.
Accordingly, it would be advantageous to develop a photodiode pixel sensor that substantially reduces dark current as well as reduces the recombination and loss of carrier charge due to scattering of electrons. In particular, it would be beneficial to develop a photodiode pixel sensor having the photodiode separated from a pixel sensor surface to reduce dark current. In addition, it would be desirable for the photodiode pixel sensor to have an architecture that increases collection efficiency of photons, particularly those generated deep within the substrate. Furthermore, it would be beneficial to develop an imaging device having improved isolation between adjacent pixels to substantially eliminate cross-collection of electrons between adjacent pixels, thereby reducing resultant image loss. Finally, it would be advantageous to be able to incorporate active semiconductor devices over a photodiode to amplify the collected carrier charge without increasing the size of a pixel to thereby increase the collection efficiency of the pixel.